C++赋值运算符重载，为什么要返回引用？
一、c/c++赋值运算符的本意为“返回左值的引用”(左值：赋值号左面的变量而非其值）
例：
int a, b = 3, c = 2;
对于a = b(a，b均为对象时)，若不返回左值的引用，将会生成临时对象。如果不处理a = b = c这样的表达式，也会正常（只是会调用拷贝构造函数和析构函数处理临时对象）。
二、为了进行连续赋值，即 x = y = z
1、赋值返回引用
x = y = z
2、赋值不返回引用
x = y = z
所以也并非必须返回引用，返回引用的好处既可以于赋值的原始语义已知，又可避免拷贝构造函数和析构函数的调用。
“题外话”：如果在类中没有说明本身的拷贝构造函数和赋值运算符，编译程序将会提供，但他们都只是对对象进行成员浅拷贝。在那些指向堆空间指针作为数据成员的类中，必须避免使用浅拷贝，而要为类定义自己的赋值运算符，以给对象分配堆内存。
PS：浅拷贝：调用系统默认的拷贝构造函数，不再新分配资源内存。深拷贝：调用自己的拷贝构造函数，分配新的资源内存。
拷贝构造函数用已存在的对象创建一个相同的新对象。而赋值运算符用已存在的对象赋予一个已存在的同类对象。
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